4 ms·

>The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation. The rocket equation doesn't account for thrust

>The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation. The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity). >If this tech lowers the weight of the first stage, it might actually RAISE the ISP of the rocket overall, even if it lowers the ISP of the engine itself. ISP is depends only on exhaust velocity. Changing the mass of the rocket cannot effect it.

amluto 8y ago
> The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity). Only sort of true. If you naively apply the rocket equation like you're imagining, you would predict a nonzero final velocity for a hypothetical rocket that has enormous ISP but thrust less than its weight. This is true if the rocket is starting out in orbit, but it's totally wrong when you're on the ground. Starting from the ground, you also have to fight gravity, so you care more about (thrust - weight) / mass.
- IshKebab 8y ago
  Sure, but "the rocket equation" is 100% talking about the classic ideal rocket equation, which I just discovered is also called the Tsiolkovsky rocket equation. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation If you want to be more specific, e.g. include gravity then you can't say "the rocket equation".
  - robotrout 8y ago
    > If you want to be more specific, e.g. include gravity > then you can't say "the rocket equation". You may not be able to say "the rocket equation" in that context, but NASA does. [1] [1] https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html https://www.nasa.gov/mission_pages/station/expeditions/exped...
    - Maybestring 8y ago
      Despite Don mentioning gravity along with deltaV he is obviously attempting to describe the classic rocket equation without scaring people away with the math.
- Maybestring 8y ago
  It is absolutely true that 'the rocket equation' doesn't account for thrust. It also doesn't account for gravity gradients, aerodynamic drag, relativity, solar wind, the cosmological constant... It's very accurate when those things contribute little, and totally inadequate for modeling spaceflight if any are significant. There are more complex versions that account for those things, but 'the rocket equation' is always understood to mean Tsiolkovsky's equation.
neaanopri 8y ago
The delta-v also depends on the mass fraction you can achieve. Removing the tanks might mean a much higher mass fraction, so you have the same delta-v with much lower Isp.

amluto 8y ago

> The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity). Only sort of true. If you naively apply the rocket equation like you're imagining, you would predict a nonzero final velocity for a hypothetical rocket that has enormous ISP but thrust less than its weight. This is true if the rocket is starting out in orbit, but it's totally wrong when you're on the ground. Starting from the ground, you also have to fight gravity, so you care more about (thrust - weight) / mass.

IshKebab 8y ago
Sure, but "the rocket equation" is 100% talking about the classic ideal rocket equation, which I just discovered is also called the Tsiolkovsky rocket equation. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation If you want to be more specific, e.g. include gravity then you can't say "the rocket equation".
- robotrout 8y ago
  > If you want to be more specific, e.g. include gravity > then you can't say "the rocket equation". You may not be able to say "the rocket equation" in that context, but NASA does. [1] [1] https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html https://www.nasa.gov/mission_pages/station/expeditions/exped...
  - Maybestring 8y ago
    Despite Don mentioning gravity along with deltaV he is obviously attempting to describe the classic rocket equation without scaring people away with the math.
Maybestring 8y ago
It is absolutely true that 'the rocket equation' doesn't account for thrust. It also doesn't account for gravity gradients, aerodynamic drag, relativity, solar wind, the cosmological constant... It's very accurate when those things contribute little, and totally inadequate for modeling spaceflight if any are significant. There are more complex versions that account for those things, but 'the rocket equation' is always understood to mean Tsiolkovsky's equation.

neaanopri 8y ago

The delta-v also depends on the mass fraction you can achieve. Removing the tanks might mean a much higher mass fraction, so you have the same delta-v with much lower Isp.